The safety device which has been broadly used in the prior art is shown in FIG. 1 to FIG. 4. Terminal stand 101 is manufactured for a fixed number of lines such as 10 lines, 25 lines, 50 lines or 100 lines. The stand 101 includes input terminals 102, output terminals 103, and terminal receiving holes 105 having connecting terminals therein, as illustrated in FIG. 2. Connection between terminals is made by solder joints, one by one, with electric wire. Accordingly, the construction of the prior art safety connector is complex and requires a great deal of labor.
Further, within the prior art safety connector 104 is a cylindrical tube 109, FIG. 3, which has a heating coil 108 wound on the outer circumference thereof. Tube 109 is secured on the upper surface of the connecting terminal 114 that is mounted on the upper surface of the inserting terminal 107. As illustrated in FIG. 4, coil 108 is serially connected in the line of the safety connector. In the upper end portion of the cylindrical tube 109 is inserted a projecting piece 110 which is secured to the tube 109 by solder of relatively low melting temperature. On the upper surface of the projecting piece 110 is mounted a discharge tube 112 which is enclosed within an electrical ground case 113. As illustrated in FIGS. 3A and 4A, ground case 113 normally is physically and electrically out of contact with connecting terminal 114. It is seen that each safety connector 104 serves two communication lines.
Spring 115 is retained in compression between ground case 113 and a ground plate 117 which is electrically and physical attached to ground terminal 116. When current below a predetermined value flows in the communication line heating coil 108, heating coil 108 of the safety connector 104 does not cause overheating and the solder that holds projecting piece 110 above the end of tube 109 is not melted.
However, when current above a predetermined value flows through heating coil 108, this overcurrent causes heating coil 108 to produce sufficient heat to melt the solder that holds projecting piece 110 in the tube 109. Accordingly, the projecting piece 110 is pushed downwardly and within the cylindrical tube 109 by the force of the spring 115 acting on ground case 113, discharge lamp 112 and the projecting piece. The circumferential surface on the lower end of ground case 113, now makes physical and electrical contact with the connecting terminal 114 see FIGS. 3B and 4B, and the overcurrent flows to ground through the path of the connecting terminal 114, ground case 113, spring 115, ground plate 117 and ground terminal 116. Accordingly, damage to the communication equipment by the overcurrent may be avoided.
Further, when overvoltage is applied in the circuit, discharge is produced within the discharge tube 112 of the safety connector 104. Thus, the grounding circuit having the electrical path through the inserting terminal 107, connecting terminal 114, projecting piece 110, discharge tube 112, ground case 113, spring 115, ground plate 117 and the ground terminal 116 is formed to provide a path to ground. Accordingly, damages to the communication equipment by the overvoltage is avoided.
In the prior art device shown in FIGS. 1 to 4, the discharge tube 112 is a cylindrical ceramic member having two facing electrodes.
Because of the production of high heat in the prior art safety devices (for example, up to about 1,600.degree. C.), damage because of fire is possible.
Upon occurrence of the overcurrent as described above, the ground case 113 comes into contact with connecting terminal 114. Accordingly, when the overcurrent or overvoltage is applied, the above-described prior art device has drawbacks which make it desirable to change the safety connector 104, or components thereof. Further, the prior art device requires testing or inspection of each of the individual circuits because direct visual inspection of the safety connector 104 is not possible. Additionally, in the prior art safety device, when dealing with tens of thounds to hundreds of thousands of lines, the required size of the safety device becomes quite large.
Because the inner wiring of the terminal stand 101 is connected by solder, the electric wire working processes are complex, difficult, and time consuming. Accordingly, production cost is high. Additionally, errors in making the wiring connections reduce the reliability of such devices. Furthermore, Korean laid-open Utility Model Publication No. 2198/1983 (Publicated Nov. 14, 1983, "Safety Device for Communication") discloses constructions in which the safety circuit is returned to the original state, together with an overvoltage protective element of the safety connector is cut off overcurrent by resistance and uses a base stand that includes printed circuitry.
In the above Utility Model Publication, by arranging printed circuitry on the base stand unit that provides the terminal stand, productability is increased and wiring errors are eliminated. However, when each of input and output terminals is, one by one, connected by solder joints on both end portions of the print circuit of the base stand unit, and in particular, when the overcurrent is caused by lightning, current above 200 A is instanteously passed. Accordingly, the printed circuitry possibly may be damaged. Further, because the overcurrent protective element that controls the flow of current is a resistance element, the fixed resistance valve has a tendency to increase with time and usage. Thus, sensitivity of the communication equipment is lowered.
On the other hand, the overvoltage protective element is deficient in security because the discharge tube is possibly burnt off the safety connector itself by the heat of discharge.